Telomerase catalyzes the addition of telomeric DNA repeats onto the ends of linear chromosomes. The essential protein and RNA components of telomerase have now been identified in many species, ranging from yeast to man. However, little is known about how these components assemble into a functional telomerase complex, or how telomerase associates with the chromosome end. The research proposed here will use a powerful combination of visual assays for telomerase localization and functional assays for telomerase activity to address these questions in the model organism S. cerevisiae. Preliminary work using fluorescence in situ hybridization (FISH) suggests that telomerase is not constitutively present at the chromosome end, in contrast to current hypotheses in the field. Instead, telomerase appears to be recruited to the telomere in a regulated process. This proposal describes approaches that will be taken to refine the FISH assay and to explore other localization methods, with the goal being to find the most efficient and informative system to study the recruitment of telomerase to the telomere. Candidate regulators of this process include the twenty or so proteins in yeast that are known to influence telomere length; the impact of these candidate regulators on telomerase activity and localization will be examined. In addition, deletion analysis of the telomerase RNA will be undertaken, to identify domains that might mediate its localization. These efforts may ultimately lead to new clinical approaches to interfere with the inappropriate telomerase activity that is seen in the vast majority of human tumor cells.